1. Field of the Invention
The invention relates generally to the field of well logging. More specifically, the invention relates to techniques for processing ultrasonic signal data to evaluate subsurface properties and tool orientation.
2. Background Art
Ultrasonic tools or instruments or calipers are one of several types of measurement tools used, including while drilling, to measure the size of a borehole. An ultrasonic tool induces a transducer that emits an acoustic signal and then detects the echo signals that are reflected from the borehole wall. The time that it takes the acoustic signal to return to the tool is proportional to the distance that the signal traveled.
A “front face echo” results from reflection of part of the acoustic signal from an interface between an acoustic transducer or sensor external window (called the “front face” of the sensor) and a fluid in the borehole. The borehole fluid or “mud” is pumped through the drill string and used to lubricate the drill bit and to carry borehole cuttings or chips upwardly to the surface as known in the art. A “formation echo” is the reflection of the acoustic signal from the formation or borehole wall. Transit time is the time it takes the signal to travel from the front face of the sensor, to the borehole wall and back again. The transit time is proportional to the distance between the tool and the borehole wall. When used with the speed of the acoustic signal in the mud, the transit time can be used to calculate the distance between the ultrasonic tool and the borehole wall. This distance is called “standoff.”
U.S. Pat. No. 4,665,511 describes an acoustic calipering apparatus for while-drilling operations. U.S. Pat. Nos. 5,852,587, 5,387,767, and Re. 34,975 (all assigned to the present assignee) describe various ultrasonic measurement apparatus and additional transducer configurations. J. J. Orban et al., New Ultrasonic Caliper for MWD Operations, SPE/IADC DRILLING CONFERENCE, paper 21947, Amsterdam, Mar. 11-14, 1991 also describes ultrasonic calipering techniques for while-drilling operations.
FIG. 1 shows a graph of the signal amplitude acquired by an acoustic transducer after actuation or “firing.” After the firing pulse, which occurs at time zero in FIG. 1, the sensor will detect ringing in the tool. Once the ringing is dampened, the transducer will detect the front face echo 12. Following the front face echo is the two-way transit time 13, during which the transducer detects substantially only noise. After the transit time 13, the transducer detects the formation echo 14.
Measurement practice known in the art includes dividing the azimuthal plane of the borehole into quadrants, each comprising about 90° of the azimuthal plane. FIG. 2 shows a diagram of the azimuthal plane 100 of a borehole 101 divided into four quadrants 201-204, or sectors. Each time the transducer fires, or for each signal acquisition, the tool 210 records the elapsed time and amplitude of the front face echo signal, the elapsed time and amplitude of the formation echo, and the quadrant 201-204 in which the tool 210 was oriented at the time of acquisition. The transit time, as previously explained, is the difference between the time at which the front face echo is detected and the time at which the formation echo is detected. At selected times, an on-board computer in the tool 210 may make a statistical evaluation of the transit time data collected by the tool 210. For each quadrant, a distribution of the frequency of particular transit times is generated.
Measurement practice known in the art also includes measuring the front face echo during a setup procedure. Before the tool is used to measure formation echo signals, it is operated and the front face echo is measured. The measurement of the front face echo transit time is assumed to be constant during actual use for that particular tool geometry. The detection of acoustic signals begins after the predetermined front face echo transit time.
For each sector, three specific transit time values are computed in the statistical analysis. The first is the average transit time for the measurements in that sector. The average is the arithmetic mean of all transit times. The other two computed transit times are called the minimum and maximum transit times. These do not represent the longest and shortest times measured, but they are values that are statistically useful for evaluating the reliability of the measurements. FIG. 3 shows a histogram of transit times with the average 301, the minimum 302, and the maximum 303. The minimum transit time 302 is defined as the transit time value where 25% of the data values have transit times shorter than the minimum 302, and 75% of the data values have transit times longer than the minimum 302. Similarly, the maximum transit time 303 is defined so that only 25% of the data values are longer than the maximum 303 and 75% are shorter. The closer the average 301, minimum 302, and maximum 303 are to each other, the better the estimate of the borehole radius. If the maximum transit time 303 and the minimum transit time 302 for a particular quadrant vary by more than about 30%, the measurement is considered non-useful.
The average 301, maximum 303, and minimum 302 are converted to transit distances by multiplying by the speed of sound in the drilling fluid. The result represents the most likely standoff in the standoff range. Again, the closer the values are to each other, the more reliable the measurement.
Prior art detection methods include filtering noise out of the detected signal. Time is divided into separate periods, each with a respective amplitude threshold signal value. A signal is not used in calculating the histogram unless it is above the amplitude threshold value for the particular time period in which it is acquired. The amplitude threshold decreases in a stair-step manner.
FIG. 4 shows an example of a conventional detection mode. There is a blanking time 410 between the time 401 the transducer fires and the first time interval 411. No signals are considered for the measurement during the blanking time. The front face echo 12 arrives during this time 410. The front face echo transit time is assumed to be the same as was measured during setup. During the first time interval 411, there is a first threshold 421. An echo signal that is received at the transducer during the first time interval 411 is considered valid if the amplitude of the signal exceeds the value of the first threshold 421. No such signals are shown in FIG. 4. Thus no signal would be detected. A second threshold 422 is applied to a second time interval 412. Any echo signal arriving at the transducer during the second time interval will not be considered valid unless its amplitude exceeds the second threshold 422. Again, FIG. 2 does not show any signal that exceeds the second threshold 422. Similarly, a third threshold 423 is used during a third time interval 413. The formation third threshold 423, thus, the formation echo would be detected. Ideally, the time periods are selected so that no echo signal would arrive after the end of the last time period, and the measurement sequence is completed. Another measurement sequence can then be commenced. Three time periods are shown in this example, but a different number of time periods can be selected to suit the needs of the measurement situation. The threshold values are determined by measuring the maximum possible contrast, i.e. PEEK/water, and using those signal values to calculate the optimum threshold. PEEK is a class of polyetherketones (see U.S. Pat. Nos. 4,320,224, 5,354,956), available from Victrex USA, Inc. of West Chester, Pa.
Prior art methods use multiple standoff measurements, but the differences, representing tool movement in the borehole, are averaged out in the calculation. Further, the threshold filtering method of the prior art can mask background noise, but it does not take into account the effect of different acoustic matching between the PEEK material and the borehole fluid.
Thus there remains a need for improved subsurface acoustic measurement techniques.